This invention relates to cooling of fluid and in particular to wet or dry air-cooled cooling towers.
Various industrial processes such as steam electric power plants have substantial need for cooling of a fluid at low temperature. When available, and if not ecologically detrimental, river water is used for cooling. Under other circumstances air-cooled cooling towers are used. The towers may be dry or wet. In a dry tower the fluid is cooled by indirect heat exchange with the air flowing over heat exchange surface. In a wet tower there is direct heat exchange between water and the air and a portion of the cooling is accomplished evaporatively.
Each of these towers furthermore may be of the mechanical or natural draft type. In a mechanical draft tower either a forced or induced draft fan produces the air flow through the tower. In a natural draft tower a very tall (in the order of 350 feet or more) tower is used with heat exchange taking place near the bottom. The tower is frequently of the hyperbolic shape and air flow is caused by the difference in density between the ambient air and the warm or saturated air within the tower.
A mechanical draft tower is less expensive to construct because of its limited size. Substantial power consumption is required however during the operation of the tower. On the other hand, a natural draft tower has a power consumption limited to that to circulate the fluid, and to pump the water in the event of a wet coolng tower. The initial cost of a natural draft tower is, however, greater than that of the mechanical draft tower.
Each of these towers suffers in its performance in the presence of wind. Normal wet cooling towers experience a phenomenon called blowout, where the air entering forcefully on one side blows straight through the lower tower horizontally taking some water with it. In a natural draft tower the tower itself blocks the wind flow thereby producing an extremely low pressure on the downwind side, reducing the air circulation. This causes a reduction in the heat transfer or thermal performance of the tower in the presence of wind.
The relatively negative pressure on the downwind side of a tower tends to create recirculation of the heated or saturated air leaving the tower into this low pressure area from which it recirculates back into the tower. This further decreases its performance.